Most HGIs are purpose-built, newer properties that reflect a rigorous standard. Most Doubletrees are random, older, often reflagged properties with few to no common elements (unless you count the cookie).

Doubletree is Hilton's "none of the above" brand. It's the column where they stick outlier or one-of-a-kind properties that are more than a Hampton Inn, less than a Hilton, too shabby or aged to be an HGI, etc.

There are superb Doubletrees in Europe (look at the former Mint properties in the UK, some of which are nicer than Conrads: modern, sleek, elegant, great service) and horrible ones in the US (the SFO Airport perimeter Doubletree is one of the saddest hotels you will ever see, inside or outside the Hilton empire: dated, musty, shabby).

Nobody knows what Doubletree is meant to stand for, not even Hilton.